If you don't spin right and have a large gear, I am sure your knees will suffer. If you ride the fixie the right way it should help those knees stay strong.

Riding my fixie everyday makes my sorry runner knees feel better.

Matt

Seems like there are two types of knee stress people could have problems with:

Impact stress - like running - the kind of thing pushing a big gear, resisting and hop skids would irritate.

Lateral stress - caused by you knee trying to move outside its usual axis of movement - the kind of thing strapping your feet into pedals or having pedals without adequate float would cause and pushing the big gear would irritate. Should be able to fix that by getting pedals that give the knee enough float to operate in its natural axis.

Lowering your gear and spinning would fix the first but not necessarily the second.

Perhaps next time he won't freeze and he'll move out of the way.

Saw off a baseball bat...and bring bear spray

I think most knee problems are already there but they don't show up until something stresses the knee, the more stress the more problems.

Let's say one leg is a little longer or your hips aren't quite even and you push normal gears and there's no problem, but then you hit the big gear and the additional stress on the knees causes them to hurt.